This invention concerns balloon dilatation procedures in which a catheter carrying a balloon on its distal end is placed within a body cavity of a patient and is inflated to dilate the cavity The procedure is commonly employed to dilate a stenosed artery, and is often performed to dilate obstructed coronary arteries U.S. Pat. No. 4,195,637 to Gruntzig discloses one type of dilatation catheter having a balloon at its distal end and adapted to perform a dilatation procedure The balloon is intended to have a maximum inflated diameter. Typically, the physician will select a catheter having a balloon with an inflated diameter corresponding to the inner diameter of the unobstructed blood vessel adjacent the stenosis to be treated. Dilatation procedures also are performed in other portions of the body, as in valvuloplasty, in which a dilatation balloon is placed and is inflated within a patient's heart valve to dilatate the commissures which have become immobile due to calcification, adhesion or other causes and to permit the valve to function more effectively.
Dilatation balloons have been made from various materials including polyvinyl chloride, polyethylene and, most recently, polyethylene terephthalate, as described in U.S. Pat. No. 4,490,421 to Levy.
It is desirable for a dilatation balloon to have several features. The balloon should have a thin wall so that it can fold down closely about the catheter shaft to a low profile, thereby enabling the deflated balloon to be inserted into and removed through narrow stenoses and passageways In heart valve dilatations, it is important that the large dilatation balloon be capable of folding down closely to the catheter shaft in a low profile, to facilitate percutaneous entry of the catheter into relatively small diameter arteries and passage of the catheter through those arteries, into the aorta and toward the heart valve to be treated as well as when removing the catheter. The balloon also should not be stiff, as stiffness detracts from the ability of the balloon portion of the catheter to bend as it is advanced through tortuous passages, a characteristic sometimes referred to as "trackability". Low stiffness (high flexibility) also is important to enable the balloon to be folded easily within the patient's body when the balloon is deflated and advanced or withdrawn. In this regard, it should be understood that the balloon when deflated, typically tends to collapse to form a pair of wings which, if not sufficiently flexible, will not fold or wrap easily about the catheter body as the deflated balloon catheter is advanced or withdrawn against body tissue. The balloon also should have a sufficiently high burst strength to enable it to impart sufficient dilatation force to the vessel to be treated. However, because different procedures and different size vessels require balloons of different diameters, the burst strength for the different balloons required for the different procedures may vary considerably. This results from the fact that the dilating force of a dilatation balloon increases as a function of the diameter of the balloon, without requiring a corresponding increase in the inflation pressure. Thus, the larger the diameter of the balloon, the lower its burst strength may be while still developing sufficient dilatation force. For example, a 20 mm diameter balloon used in a valvuloplasty procedure need only have a burst strength of about 3 to 6 atm whereas a 3 mm balloon used in the dilatation of small coronary arteries may require a burst pressure of 10 to 20 atm. Larger diameter balloons do not require as much internal pressure to create the same dilating force, therefore, there are many dilatation balloons in which high burst strength is not advantageous.
Another desirable feature of dilatation balloons is that they should be dimensionally stable, that is, they should retain their size and structural integrity during storage and, when inflated, should not exhibit excessive radial expansion beyond the nominal inflated diameter.
The dilatation balloons of the prior art have been deficient in one or more of the foregoing characteristics. For example, Levy U.S. Pat. No. 4,490,421 relates to a balloon intended to have a high burst pressure with low radial expansion when placed under pressure. Balloons made according to the Levy '421 patent, however, are neither sufficiently thin walled nor flexible, and are not sufficiently dimensionally stable during inflation or when stored. The balloons may shrink during storage and, consequently, will not inflate to the nominal diameter expected by the physician and required for the particular procedure.
It is among the general objects of the invention to provide improved dilatation balloons and a method for their manufacture which provides superior properties of thin walls, flexibility and high strength as well as full dimensional stability both in storage and when inflated.